optical flat
A disk of high-quality quartz glass having at least one side ground and polished with a deviation in flatness less than 50 nanometers all over, and a surface quality of 5 microfinish or less; used, in an interferometer, to measure the flatness of other surfaces.
optical flat: a reference standard for measuring surface flatness
An optical flat is a disk of optical glass, typically fused silica or borosilicate, with one or both surfaces polished to extreme flatness. The working surface deviates from true flatness by less than 50 nanometers across its entire area, making it a precision reference standard rather than a measuring instrument in itself. It is placed in contact with a test surface and illuminated with monochromatic light, usually a helium-neon laser at 632.8 nanometers or a sodium lamp at 589 nanometers, to reveal flatness deviations via interference fringes.
When light passes through or reflects from the gap between the optical flat and the test surface, constructive and destructive interference create alternating bright and dark bands called Newton's fringes or interference fringes. Each fringe represents a height difference of half a wavelength of the illumination used. By counting and analyzing the fringe pattern, an operator can determine how far the test surface deviates from perfect flatness. A perfectly flat surface produces perfectly straight, evenly spaced fringes; any irregularity in the surface under test distorts the fringe pattern proportionally.
Optical flats come in several standard diameters, typically 25 mm to 150 mm, chosen to match the size of the surface being tested. Thicker flats, usually 12 to 25 mm, resist warping under their own weight and thermal stress better than thin ones. The surface finish specification of 5 microfinch or better means the roughness is measured in millionths of an inch and is polished smooth enough that light can reflect coherently. Wringing, a technique where light pressure and a drop of immersion oil create molecular adhesion, may be used to hold an optical flat temporarily in position during measurement.
Common problems and wear
Optical flats are fragile and degrade with use. Dust particles, fingerprints, and scratches scatter light and destroy fringe visibility. Any direct contact between the flat and a rough surface will damage the polish and render the flat unusable. Temperature changes cause the glass to expand or contract unevenly, introducing temporary flatness errors. A flat that has been dropped, stored improperly, or used on abrasive materials may develop a slight permanent warp called warpage, making it no longer suitable as a reference standard.
The term optical flat originated because the device creates the optical phenomenon of interference, and the flatness of its surface is its defining quality. It is a passive standard, not an active measuring tool; the flat itself is measured against a master standard during manufacture and certification. In production and quality control, optical flats are essential for inspecting gauge blocks, lapping plates, precision spindle components, and any surface where flatness is critical to function. A worn or uncertified optical flat will produce misleading fringe patterns and invalid measurements, so frequent recertification against a known master flat is necessary in high-accuracy work.